Octopamine receptors on Aplysia neurones mediate hyperpolarisation by increasing membrane conductance

Abstract

OCTOPAMINE is a phenylethylamine synthesised by β-hydroxylation of tyramine and differs from noradrenaline in lacking one hydroxyl group on the phenyl ring. Although Aplysia nervous tissue, like that of most molluscs, contains little or no noradrenaline^1,2, it has between 0.20 and 1.85 pmol octopamine per mg of tissue in the various ganglia. Individual identified neurones also contain detectable octopamine and in neurone R14 it reaches 1.5×10⁻⁴ M (ref. 2). Although octopamine-activated adenyl cyclases have been found in Aplysia nervous tissue³ and insects^4,5, there has been no electrophysiological demonstration of specific octopamine receptors. We now report receptors in the nervous system of Aplysia that are most sensitive to octopamine and mediate a hyperpolarising conductance-increase response. This provides further evidence that octopamine can function as a neurotransmitter.